1. Field of the Invention
The present invention relates to devices optimizing the states of charge of a plurality of cells that form an assembled battery, and assembled battery systems including the devices.
2. Description of Related Art
In recent years, an assembled battery has been widely used, for example an assembled battery including a plurality of lithium-ion secondary cells connected in series is used in a hybrid vehicle as a power source. Discharge power of an assembled battery is limited by a cell with the lowest state of charge (SOC) among a plurality of cells that forms the assembled battery. Therefore, performance of an assembled battery decreases due to the variation of SOC of the plurality of cells that form the assembled battery.
Accordingly, a process to equalize the SOCs of the plurality of cells of the battery is required. In a conventional equalizing process, the voltages across the respective cells that form an assembled battery (open-circuit voltages) are detected, and the lowest value or the average value of the detected cell voltages is used as a target value for the equalizing. Thus, electric discharge is conducted on the cell whose voltage is above the target value for equalizing, thereby equalizing the SOCs of the plurality of cells that form the assembled battery.
For example, as shown in FIG. 10 in which three cells 1 to 3 form an assembled battery, when the SOC of the cell 1 is the lowest before equalizing, the cells 2 and 3 are discharged to the SOC of the cell 1, thereby equalizing the SOC of the cells 1 to 3.
However, the inventor of the present invention has discovered in his research that the full charging capacities of the plurality of cells that form the assembled battery vary due to the variation as-manufactured, or the variation attributed to temperature distribution when used, and therefore, the conventional equalizing process cannot sufficiently elicit the performance of the assembled battery.
For example, as shown in FIG. 11, in which three cells 1 to 3 having different full charging capacities form an assembled battery, in the case where the cells 1 to 3 are equalized when the SOC of the cells is approximately 70 percent, since the discharge amounts per unit time of the cells 1 to 3 are uniform, when the SOC of the cell 1 reaches 50 percent due to discharge of the assembled battery after the equalization, the SOC of the cells 1 to 3 varies.
Also, in the case where the cells 1 to 3 are equalized when the SOC of the cells is approximately 30 percent, since the charge amounts per unit time of the cells 1 to 3 are uniform, when the SOC of the cell 1 reaches 50 percent due to charge of the assembled battery after the equalization, the SOCs of the cells 1 to 3 vary.
The discharge characteristic of the battery decreases as the SOC decreases, and the charge characteristic decreases as the SOC increases as shown in FIG. 12. Therefore, the higher performance of the assembled battery can be obtained when the SOCs of the plurality of cells are equalized with the SOC in which both the discharge characteristic and charge characteristic are favourable and well balanced, such as approximately 50 percent, rather than with a high or low SOC.
With the conventional equalizing process, in the case where the SOCs of a plurality of cells are equalized when the SOC is approximately 70 percent as described above, for example, since the SOCs of the cells vary when the SOC of the cells decreases to around 50 percent due to discharge of the assembled battery after the equalization, the performance of the assembled battery cannot be sufficiently elicited.
Accordingly, the equalization with the SOC of the plurality of cells of around 50 percent is considered. It is desirable to equalize the assembled battery of a hybrid car or the like when the car is stopped and the battery is not charged or discharged in order to accurately measure the amount of charge. However, the hybrid car or the like is not necessarily stopped when the SOC of the assembled battery is around 50 percent. It is therefore problematic to do the equalization when the car is stopped in that the equalization is done very infrequently.